1. Field of the Invention
The present invention relates to an electromagnetic wave detection element. The present invention particularly relates to an electromagnetic wave detection element that accumulates charge generated by irradiating electromagnetic waves for detection onto plural pixels disposed in a matrix, and detects the accumulated charge amounts of charges as information representing an image.
2. Description of the Related Art
Radiation electromagnetic wave detection elements such as FPDs (flat panel detectors), in which an X-ray sensitive layer is disposed on a TFT (thin film transistor) active matrix substrate and that can convert X-ray information directly into digital data, and the like, have been put into practice in recent years. Such electromagnetic wave detection elements are used in radiographic imaging apparatuses. As compared with a conventional imaging plate, an image can be confirmed immediately at an FPD. Further, the FPD has the advantage that video images as well can be confirmed. Therefore, the popularization of FPDs has advanced rapidly.
Various types of electromagnetic wave detection elements are proposed. For example, there is a direct-conversion-type electromagnetic wave detection element that converts radiation directly into charges in a semiconductor layer, and accumulates the charges. Moreover, there is an indirect-conversion-type electromagnetic wave detection element that once converts radiation into light at a scintillator of CsI:Tl, GOS (Gd2O2S:Tb), or the like, and, at semiconductor layer, converts the converted light into charges and accumulates the charges.
In electromagnetic wave detection elements, for example, plural scan lines and plural signal lines are disposed so as to intersect with each other, and charge storage capacitors and TFT switches are provided at each of the intersections between the scan lines and the signal lines. Further, in electromagnetic wave detection elements, a semiconductor layer is also provided to cover the charge storage capacitors, and the TFT switches are provided at each of the intersections. When radiographic imaging apparatuses using such electromagnetic wave detection elements capture radiographic images, during irradiation of X-rays, an OFF signal is output to each of the scan lines, and each of the TFT switches is switched OFF. Accordingly, the charges generated in the semiconductor layer are accumulated in each of the charge storage capacitors. Then, when reading out an image from the radiographic imaging apparatus, an ON signal is output to each scan line one at a time in sequence, the charges accumulated at each of the charge storage capacitors is read out as an electrical signal, and the read-out electrical signal is converted into digital data. The radiographic imaging apparatus obtains radiographic images according to the above procedures.
However, when radiographic imaging apparatuses attempt to read out radiographic images successively from the electromagnetic wave detection elements in order to obtain a video image, the number of frames of radiographic image read-out per second (the frame rate) is large. Further, in a radiographic imaging apparatus, the scanning time for outputting an ON signal to a single scanning line, and for reading out the electrical signal, also gets smaller as the number of scan lines of the electromagnetic wave detection element increases.
The scan time 1 H can be derived according to expression (1) below, where the frame rate is denoted by FR, and the number of scan lines of the electromagnetic wave detection element is denoted by Gn.1H=1/FR/Gn  (1)
For example, when the frame rate FR is 60, and the number of scan lines Gn is 1000 lines, then the scan time 1 H is 16.7 μs.
If a TFT active matrix substrate is used as a liquid crystal display (LCD), the scan time 1 H can be used entirely as time for reading in data. This means that 16.7 μs is a sufficient time.
However, if a TFT active matrix substrate is used as a detection element that captures an image, then in order to perform noise reduction (namely, when used as a detection element for medical radiographic image capture that requires low-noise imaging), not all of the scan time 1 H can be allocated to a charge read-out period, since after amplifying the electrical signal of charge that has been accumulated in each of the charge storage capacitors, and is converted into digital data in an A/D (analogue/digital) conversion unit, and correction processing and the like is performed to the converted digital data. This makes it difficult to image video images of high frame rate.
Japanese Patent Application Laid-Open (JP-A) No. 2003-2642273 (document 1) discloses a technology to address this issue, enabling imaging of video images of high frame rate. In the technology disclosed in document 1, the image receiving face of the electromagnetic wave detection elements is divided into plural pixel areas, read-out devices are provided for each of the pixel areas, and image read-out is performed for each of the pixel areas.
However, in the technology of document 1, the image receiving face is divided into plural pixel areas and pixels are read out in series from each of the pixel areas. Thus, although the techniques disclosed in Document 1 may increase the speed of pixel read-out overall, they do not increase the speed of pixel read-out for a single pixel area.